This invention is directed to an analog-to-digital converter, and in particular to an analog-to-digital converter wherein the converter is synchronized with the timing of the counter circuit, the counter circuit utilizing a quartz crystal vibrator or other like time standard as a time base for effecting conversion.
Heretofore, analog-to-digital converters including counter circuits for producing a digital signal in response to an analog voltage thereto, have utilized a gating circuit between the oscillator circuit and the counter circuit for starting and terminating the count of the counter. The gating circuit is usually controlled by an integrator circuit adapted to sum an analog voltage, and a reference voltage of an opposite polarity to the analog voltage, and in response to summing same to zero, the gating circuit terminates the count of the counter to produce a digital representation of the analog signal. Such analog-to-digital converters are less than completely satisfactory in view of the use of the signal from the integrator circuit as a gating signal. Such use of the integrated signal as a gating signal and a quartz crystal vibrator as a time standard causes the presence of AC noise voltages superposed upon the analog voltage to be substantially unavoidable.
In the art, it is recognized that the time over which a voltage to be converted is integrated must be set at "N" times the frequency of the superposed noise signal (where "N" is an integer). By adjusting the oscillating frequency of the oscillator circuit such setting is achieved over a wide range of frequencies. Accordingly, the oscillator circuit must be one which is stable over a wide range of frequencies thereby eliminating the use of inexpensive quartz crystal vibrators of the type which are mass produced and utilized in color televisions, quartz crystal timepieces, etc. Instead, oscillator circuits having adjusting capacitors of considerably large range are required to effect the adjustment over the range of frequencies necessary thereby increasing the cost of such analog-to-digital converters even when integrated circuit techniques are utilized to fabricate same. An digital-to-digital converter, wherein mass-produced quartz crystal vibrators can be utilized, and wherein AC noise signals superposed upon the analog signal are substantially eliminated is provided in accordance with the instant invention.